Estimating body segment orientation by applying inertial and magnetic sensing near ferromagnetic materials

Inertial and magnetic sensors are very suitable for ambulatory monitoring of human posture and movements. However, ferromagnetic materials near the sensor disturb the local magnetic field and, therefore, the orientation estimation. A Kalman-based fusion algorithm was used to obtain dynamic orientations and to minimize the effect of magnetic disturbances. This paper compares the orientation output of the sensor fusion using three-dimensional inertial and magnetic sensors against a laboratory bound opto-kinetic system (Vicon) in a simulated work environment. With the tested methods, the difference between the optical reference system and the output of the algorithm was 2.6$^circ$ root mean square (rms) when no metal was near the sensor module. Near a large metal object instant errors up to 50$^circ$ were measured when no compensation was applied. Using a magnetic disturbance model, the error reduced significantly to 3.6$^circ$ rms.